BAGHDAD/BARTELLA, Iraq (Reuters) - U.S.-backed Iraqi forces moved closer on Wednesday to a town south of Mosul where aid groups and regional officials say Islamic State has executed dozens of prisoners.

A military statement said security forces advanced to the edge of Hammam al-Alil, a thermal water resort, after an elite unit breached the eastern limits of Mosul, the ultra-hardline group's last major city stronghold in Iraq.

The battle that started on Oct. 17 with air and ground support from a U.S.-led coalition is shaping up as the largest in Iraq since the U.S.-led invasion of 2003.

Mosul still has a population of 1.5 million people, much more than any of the other cities captured by Islamic State two years ago in Iraq and neighboring Syria.

The United Nations cited reports on Tuesday that Islamic State, which is also known as ISIL, is attempting to displace Hammam al-Alil's estimated population of 25,000 for use as human shields and protection against air and artillery strikes.

"We have grave concerns for the safety of these and the tens of thousands of other civilians who have reportedly been forcibly relocated by ISIL in the past two weeks," U.N. human rights spokeswoman Ravina Shamdasani said.

The town, 15 km (9 miles) south of Mosul, had a pre-war population of 65,000, a local official said.

Aid organizations, local officials and Mosul residents have cited reports that IS has executed dozens of people in Hammam al-Alil and barracks nearby on suspicion of planning rebellions in and around Mosul to aid the advancing troops.

Abdul Rahman al-Waggaa, a member of the Nineveh provincial council, told Reuters last week that most of the victims were former police and army members. The men were shot dead, he said, quoting the testimony of remaining residents of the villages and people displaced from the area.

Security forces advancing north on the western bank of the Tigris River recaptured five villages on Wednesday, the closest of them just 5 km (3 miles) from Hammam al-Alil, according to military statements.

Just across the river from those forces lie the ruins of the ancient Assyrian city of Nimrud, which the Iraqi government says was bulldozed last year as part of Islamic State's campaign to destroy symbols which the Sunni Muslim zealots consider idolatrous. Army troops heading north on that side of the Tigris have yet to reach the area.

FEARS OF A HUMANITARIAN CRISIS

The United Nations has said the Mosul offensive could trigger a humanitarian crisis and a possible refugee exodus if the civilians inside in Mosul seek to escape, with up to 1 million people fleeing in a worst-case scenario.

The International Organisation for Migration said nearly 21,000 people have been displaced since the start of the campaign, excluding thousands of villagers taken into Mosul by retreating jihadists who used them as human shields.

Elite Counter Terrorism Service (CTS) troops were the first to breach Mosul's official boundary this week. They said on Tuesday they were in control of the state television station.

A CTS commander, Lt. General Abdul Ghani al-Assadi, told reporters in Bartella, a village east of Mosul, that the unit will pause its advance on the eastern front because of some rainfall early on Wednesday.

"God willing the next stage will begin within hours. This depends on the weather," he said.

A resident of Mosul's eastern Karama district, still under Islamic State control, said on Wednesday four people were killed in artillery fire and air strikes, and said he saw two partially destroyed houses.

"There's a compulsory curfew in our area. The women and children are frightened and the men can't do anything. We can't even help the wounded," the resident told Reuters by telephone.

"One person was wounded by shrapnel in his house and he bled to death. No one could save him. We can't retrieve the bodies. Anyone who leaves his house risks his life."

However residents said the city was quieter than on Tuesday.

Assadi said a curfew had been imposed on the recaptured eastern suburb of Kokjali to protect residents from mortar bombs fired by the insurgents.

Kurdish peshmerga fighters are also deployed on the eastern and northern fronts, and Iranian-backed Iraqi Shi'ite militias are attacking Islamic State west of Mosul.

TRIBAL MILITIAS

Some Sunni Muslim volunteers opposed to Islamic State also want to join the battle. Amnesty International said on Wednesday fighters from one group, the Sab'awi tribal militia, tortured and humiliated men and boys in villages southeast of Mosul who were suspected of having ties to Islamic State.

"There is strong evidence that Sab'awi tribal militia members have committed crimes under international law by torturing and otherwise ill-treating residents... in revenge for crimes committed by IS," Amnesty's Lynn Maalouf said.

In one incident, seven men and boys were placed in poultry cages in the middle of a public roundabout. They were brought out one by one, forced to say they were donkeys, and were beaten, Amnesty said.

There have been no reports of abuses by pro-Iranian militias, but their involvement is causing alarm in Turkey which has had troops deployed north of the city since last year to train and support some of the Sunni Arab volunteers.

The Turkish army has begun deploying tanks and other armored vehicles to the town of Silopi near the Iraqi border.

Turkey says it has a responsibility to protect ethnic Turkmens and Sunni Arabs in the area around Mosul, once part of the Ottoman Empire. It fears both Kurdish PKK militants and Shi'ite militias, which the Iraqi army has relied on in the past, will be used in the campaign and stoke ethnic bloodletting.

Iraqi Prime Minister Haider al-Abadi said after a cabinet session in Baghdad on Tuesday that tensions with Turkey have eased in the last week but that Iraq would respond to any "violation" of its territory.